1. Field of the Invention
This invention relates generally to direct current power control circuits. This invention applies anytime a DC battery is used in an application requiring less than constant full battery discharge and, more specifically, to circuits for controlling the speed of motors associated with DC battery powered motors or vehicles.
2. Description of the Related Art
The most common method of regulating the speed of a DC electric motor typically involves placing a variable resistor or a sequence of discrete resistors in series with the windings of the electric motor. While this method does provide sufficient speed control of the motor it has two distinct disadvantages. First, the power drawn from the battery in such an arrangement is not efficiently reduced in direct proportion when the speed of the motor is reduced. This is because a portion of the power is dissipated through the resistors rather than entirely through the motor. The same current drain occurs on the battery whether the motor is run at high speed or low speed. The only change is in the relative distribution of the load between the resistors and the motor windings. In some applications the current drain does decrease as the motor decreases speed however the power efficiency of the drain also decreases. A second disadvantage which is a by product of the first, is that the power dissipated through the resistors is given off as heat, which besides being a waste of energy, can be a problem for some applications.
One attempt at a solution to the problem is to utilize solid state switching devices to control and regulate the current flow to the motor windings. One type of solid state switching device capable of handling the currents typically required by large electric motors is the SCR (silicon control rectifier). SCRs function much as a mechanical switch might by allowing a large current to flow between two points when a relatively small voltage is present to toggle the rectifier. In a solid state circuit a SCR can be pulsed so as to "chop" the current through an electric motor thereby regulating the current flow and, thus, the motor speed. While SCRs do have lower energy losses than the resistor control arrangements they still result in a substantial dissipation of power in the form of heat. SCRs are also relatively large solid state components and are often high in cost. SCRs additionally suffer the drawback of characteristically poor load sharing when placed in parallel across a large load.
A more promising solid state device that to all appearances functions in much the same way as an SCR is the metal oxide semiconductor field effect transistor or MOSFET. Like the SCR a MOSFET allows a relatively large current to flow in a circuit when a relatively small "gate" current toggles it "on". Unlike the SCR a MOSFET enjoys the advantages of having very high input impedance, good load sharing propensity, small size, low cost, and very fast switching times.
The fast switching times of MOSFET devices allow them to function at very high frequencies compared to the slower SCRs. This creates the added advantage of operation at a level that is above the range of human hearing and therefore allows relatively quiet control of the electric motor. The higher frequencies do however have the disadvantage of creating larger voltage spikes when the MOSFETs are switched on and off. This would counsel for the use of MOSFETs with higher voltage ratings to handle these spikes, but with the higher voltage ratings come higher internal resistances and lower current capacities. One solution to this problem is to provide external freewheeling diodes to act as transient suppression devices across the load.
Another attempted solution is described in Post, U.S. Pat. No. 4,626,750. Post's attempted solution is to use an electronic circuit chopper control system to control groups of parallel field effect transistors and parallel power diodes. The power diodes are distributed apart from each other and among each group of field effect transistors. Post attempts to reduce voltage spikes caused by the field effect transistors being turned on and off by using the power diodes to draw counteracting current from the battery. Unfortunately, this arrangement results in excess heat which must be dissipated through fins, which in turn results in less efficient power drain from the battery. Post also requires a complex control circuit to ensure that the system operates safely within acceptable parameters so as to not damage the electronic components or host vehicle.